(1) Field of the Invention
The present invention relates to a system for exhaust gas purification, capable of effectively purifying the harmful substances present in the exhaust gas emitted from an internal combustion engine, particularly the hydrocarbons discharged from the engine in a large amount during the cold start.
(2) Description of Related Art
Active researches and developments have heretofore been made on systems for exhaust gas purification, used for purifying the harmful substances present in exhaust gas from automobile engines, such as nitrogen oxides (NO.sub.x), carbon monoxide (CO), hydrocarbons (HC) and the like. In recent years, as the regulation on exhaust gas has become stricter, the purification of HC during engine start (cold start) has become an important technical task.
The reason is as follows. During the cold start of engine when the temperature of exhaust gas from engine is low, the catalyst disposed in exhaust pipe of engine does not reach its light-off temperature and has a low purification ability. Moreover, during this period, as compared with the period of continuous operation of engine, a large amount of HC is discharged from the engine. As a result, the HC discharged during the cold start occupies a large proportion of the total harmful substances discharged from the engine and needs to be removed.
As one means for achieving the above-mentioned technical task, there has been proposed a system for exhaust gas purification disposed in the exhaust pipe of an internal combustion engine, comprising not only a catalyst but also zeolite (as adsorbent) having an adsorbability for HC. In this system, zeolite is used to adsorb a large amount of the unburnt HC discharged from the engine during the cold start, temporarily for the period from the start of catalyst heating to the activation of catalyst.
In the above system for exhaust gas purification using zeolite, its performance varies greatly depending upon the HC adsorption capacity of the zeolite used. Also, the HC amount discharged from an internal combustion engine during the cold start varies greatly depending upon the engine used. Consequently, the performance of an exhaust gas purification system using a zeolite having a certain HC adsorption capacity varies greatly depending upon the engine in which the system is used. Therefore, in order to effectively adsorb and purify a large amount of the HC discharged from an engine during its cold start, it is necessary to allow the exhaust gas purification system disposed in the engine, to contain a zeolite having an adsorption capacity allowing for effective adsorption of the HC discharged from the engine during a certain period of cold start.
However, there is hardly known a system for exhaust gas purification in which sufficient consideration is made of the relation between the amount of HC discharged during cold start and the HC adsorption capacity of zeolite.